Thanks to genetic progress, the milk potential of dairy cows has reached record levels. High milk yield means a large energy deficit. To close that gap, especially in early lactation, farms increase the share of starch and concentrate feed (grain, molasses, etc.) in the ration. This is where one of the most insidious and costly losses on dairy farms appears: subacute ruminal acidosis (SARA).
Unlike clinical acidosis, SARA does not put the cow down and does not produce an obvious disease picture. Even so, it spreads through the herd and causes milk fat depression, liver abscesses, hoof inflammation (laminitis) and unexplained breeding problems. In this guide we explain how SARA develops, how to spot it in the field and how to prevent it.
What Is Subacute Ruminal Acidosis (SARA)?
The rumen is a living fermentation tank of about 150-200 litres, home to billions of bacteria, protozoa and fungi. For this ecosystem to work well, rumen fluid pH must stay between 6.2 and 6.8.
When fast-fermenting carbohydrates (corn, barley, wheat meal) are eaten in excess, starch-digesting bacteria (e.g. Streptococcus bovis) multiply rapidly and produce large amounts of volatile fatty acids and lactic acid. Rumen pH that stays below 5.8 for more than 3 hours a day is defined as SARA.
If pH falls below 5.5, the fibre-digesting cellulolytic bacteria (Fibrobacter succinogenes, Ruminococcus albus) become inactive. Cellulose breakdown slows, acetate production drops and milk fat synthesis suffers serious losses.
Critical threshold: pH 5.8. Every hour that rumen pH spends below this value puts the cellulolytic microflora at risk.
The 5 Main Signs of SARA in the Herd
Because SARA is subclinical (no visible symptoms), the cause is often blamed on feed quality or the season. These 5 indicators should raise the alarm during herd monitoring:
- Drop in milk fat: Bulk tank fat below 3.5%, and as low as 3.0% in high-yielding herds. A milk fat to milk protein ratio below 1.0 is a typical sign of SARA.
- Erratic dry matter intake: Cows eat eagerly one day and avoid feed the next. In acidosis the animal stops eating, then overeats when pH recovers, creating a vicious cycle.
- Inconsistent manure and undigested feed: Loose, bubbly, shiny yellowish and mucus-filled manure, with plenty of undigested coarse fibre and corn kernels in the manure sieve.
- Hoof lesions (aseptic laminitis): Endotoxins released from the cell walls of bacteria that die in the acidic environment, together with histamine, enter the blood. They cause hoof tissue inflammation, sole ulcers and lameness.
- Liver abscesses and slaughterhouse losses: Acid erodes the rumen papillae (ruminitis). Fusobacterium necrophorum passes through the micro-wounds into the blood and reaches the liver, where it forms abscesses.
Healthy Rumen vs. SARA Rumen
In a healthy rumen:
- Average daily pH is stable between 6.2 and 6.8.
- The acetate to propionate ratio is above 3:1.
- Bulk tank milk fat is 3.6-4.2%.
- Rumination time is 450-550 minutes per day.
- The manure sieve shows well-digested, homogeneous fibre.
- Lameness and sole ulcers affect less than 5% of the herd.
In a rumen with SARA:
- pH stays between 5.5 and 5.8 for more than 3 hours a day.
- The acetate to propionate ratio falls below 2.2:1.
- Bulk tank milk fat drops to 2.8-3.3%.
- Rumination time is below 380 minutes per day.
- Whole corn kernels and coarse fibre show up in the manure sieve.
- Lameness can exceed 18% of the herd.
The economic result is lost fibre digestion, lost milk price premiums, poorer feed efficiency and cows leaving the herd early.
Preventing SARA: Physical Fibre, Buffers and Live Yeast
The first rule of preventing SARA is to maintain physically effective fibre (peNDF) in the ration. Forage particle size should be checked with Penn State sieves, and rumination should be encouraged to maximise natural saliva (a source of sodium bicarbonate).
In high-yielding rations, managing physical fibre alone is not enough. Chemical and biological support is also needed:
- Dual-phase rumen buffers: Sodium bicarbonate dissolves quickly and stops sudden pH drops. Magnesium oxide (MgO) and calcium carbonate release slowly and provide longer-lasting buffering.
- Live yeast (Saccharomyces cerevisiae): Live yeast cells consume dissolved oxygen in the rumen and strengthen the anaerobic environment. They have been reported to support beneficial bacteria such as Megasphaera elsdenii and Selenomonas ruminantium, which convert lactic acid to propionate. Lactic acid is then used up before it accumulates.
Frequently Asked Questions
What is the difference between SARA and acute acidosis?
In acute acidosis, rumen pH falls below 5.0; the animal stops eating completely and may show severe diarrhoea, dehydration, recumbency and even death. In SARA, pH is between 5.5 and 5.8. The animal stays on its feet but quietly loses milk fat, goes lame and struggles to conceive.
Is a drop in milk fat always a sign of SARA?
SARA is the most common cause of milk fat depression, but excess unsaturated fatty acids in the ration (trans fatty acid formation) or a lack of fibre in the forage can also lower milk fat. The diagnosis should be confirmed with rumen pH measurement and a manure sieve.
How much baking soda (sodium bicarbonate) should go in the ration?
In a standard lactating cow ration, sodium bicarbonate is typically used at about 0.75-1.0% of dry matter intake (roughly 150-250 grams per cow per day). Soda alone may not be enough; it should be planned together with magnesium oxide and live yeast. Always set the dose with your ration nutritionist.
Does live yeast stay alive in pelleted feed?
Standard yeasts largely lose viability under the 70-85 °C heat and steam of the pellet press. For pelleted feeds, micro-encapsulated or heat-tolerant (thermotolerant) yeast forms should be preferred. MCB Vital premixes use strains of this kind.
